Understanding & Live Graphing of the Rear O2 Sensor: OBD4Everyone Ep.19

On this episode of OBD 4 Everyone
we are going to explain and show everything you need to know about the rear o2 sensor AND show you how to use OBD Fusion LIVE graphing feature.

Almost every gasoline fueled car made in the last 25 years will have at least two o2 sensors located in the exhaust system.

One is located BEFORE the catalytic converter and it's main job is to provide feedback to the engine control unit on the exhaust gas concentration of oxygen.

Then the ECU will adjust the air/fuel mixture to maximize catalytic converter's efficiency and minimize emissions.

Now, the other o2 sensor is located after the catalytic converter, and this is the one we are going to be talking about.To make sure we all have the same understanding, there are multiple names for the rear O2 sensor
for example:

• downstream o2 sensor
• post cat o2 sensor
• and O2 sensor 2.

All of these names simply tells us it's located AFTER the catalytic converter.

If the air/fuel mixture is greater than 14.7 :1, or lean, the output voltage will be around 0.1 volts.

If the air/fuel mixture is less than 14.7 :1, or rich, the output voltage will be around 0.9 volts

and IF the air/fuel ratio is 14.7:1, the output voltage will be close to 0.5 volts.

This "narrow band" of sensitivity is why it's also known
as a "Narrow band O2 sensor"

Now, to determine how well the catalytic converter is working,
the ECU measures just about everything it can from the rear O2 sensor
like:

• how much time it takes to switch from rich to lean
• how much time it takes to switch from lean to rich
• the frequency of how often it switches
• and there's more, but I think you get the idea.I'll bet you are wondering, why does the air/fuel mixture change so much?
shouldn't it be more stable?

A few things come into play here...
When we accelerate, the mixture will be rich.
When we lift off the gas pedal to slow down, the mixture will be lean.

and when idling or driving at a constant speed
the ECU will change the air/fuel ratio back and forth between slightly rich and slightly lean.

while it analyzes how the rear o2 sensor responds to these changes.

A healthy catalytic converter has a high capacity to store oxygen,
and as a result of this, the rear o2 sensor should have a low switching frequency.

If it has a high switching frequency, the catalytic converter oxygen storage is low and it is not functioning as it should.

When the switching frequency is too low, the check engine light is turned on with a P0420 or P0430 which means Catalyst system
efficiency below threshold.Alright, let's wrap up this episode...

I hope you now have a better understanding of what the rear o2 sensors do and what their output should look like for different operating conditions.

And since most rear o2 sensors are the narrow band type,
everything shown in this video should apply to any gasoline fueled car
made in the last 25 years.

As always, thank you for watching and please subscribe.

[scottpear]

Man Ive been a heavy line auto mechanic for 25 years plus years and that is the best information I've come across. very easy to understand and that was great as I needed a refresher

[tbjylud]

I've got a 2012 Toyota Corolla with about 115K miles on it. It has been well taken care of over the years by the previous owner, they changed the oil and such impeccably, had everything done on time, etc. As of the past couple of years, the downstream O2 sensor started testing only 1 or 2 out of the total of 5 readiness tests on OBD2, flipping between 1 and 2 out of 5 total tests for it and basically just halfway testing and untesting. I've checked voltage on the wiring, changed the O2 sensor 3 times in 2 years (Toyota-branded Denso made no difference vs aftermarket), had the battery tested, etc. When the ECU is reset completely, virtually all tests are complete the next day. It requires overnight testing for the Evap, but everything else tests very rapidly, including both readiness tests for the upstream engine-side O2 sensor. The catalyst also isn't testing, which I suspect is because of the downstream O2 sensor (before/on the catalyst) not testing first.

What could cause this? There is no check engine light, no error codes in the scanner or anything to indicate that anything is wrong. What am I missing? OBD2 device also shows the voltage of the downstream O2 sensor is registering and also it is reading the catalyst temperature properly. So I see that it is getting about 1V when the throttle is pressed, the upstream and downstream are talking to one another by adjusting fuel trim and such based on whether the throttle is being pressed, etc. Toyota recommended cleaning the throttle body, but that doesn't seem to be related to the downstream O2 sensor that we're having issues with. It's simply jumping back and forth between 2 of 5 readiness tests completed, then back to 3 or 4 out of 5 tested, then back to 2 or 3 tested and just says they haven't been completed yet. We've driven it hundreds of miles after reset. Same thing over again.

[mattbrown]

This video is insanely high quality. I really hope your content gets the visibility it deserves.

[The_Allstar]

Great video! Really learned a lot.
My O2 S2 Sensor switches between 0, 53V and 0, 67V at idle with about 1.7s peak to peak frequency.
Not sure, but might be a bad catalytic convertor then.

I do have very rare rpm drops when approaching a light slowly to 500rpm and sometimes backfires at high rpm. :(

[sylvesterfurtak]

Truly great info. My question is: I have 2003 Land Rover Freelander 2.5L V 6 engine. It has also two cat. converters, two Front and two Rear O2 sensors as explain in the show. Last year my local shop cut out completly one of my cat. converters and weld in just a straight pipe instead and then cut out the end of the second cat and pierced trough a hole with a screwdriver (because they couldn't access its front) to cut completly out. They also installed some kind of adapters?? to correct check engine light issue. However, right now my check engine light comes on all the time after a few minutes of driving. Also, they told me that I have to remap my ECU to eliminate Rear O2 sensors so the check engine light won't come back again. I know this will be very risky procedure as ECU remapping can go wrong during. and my car won't derivable. What I should do to correct this situation? Are they are any solutions to correct this issue? Please let me know if you can. Thank you very much. S.F.

[EMRDJ]

i have purchased the extended obf fusion for my rav4 2019 adventure, i sat the graph as per ur instructions and when in action the only grph that was working is the accelerator and the speed, the o2 sensors are not showing any graph though the engine temperature is above F180, the interface am using is ELM327 wifi, the reason am running this diagnosis is that when i drive for two hours or more i start hearing explosions in the exhaust, it might be also important to mention that the fuel we use most of the time is not good and stations mix it with water.

[boogerjoe]

2:48 "when the switching frequency becomes too low the computer throws the p0420 code." This should be when the switching frequency becomes too HIGH the computer throws the p0420 code, or it could be when the capacity to store oxygen becomes too low the computer throes the p0420 code

[davidkinasevych]

Really good video! Thanks for explaining this so well.

[rexanderson]

I have an Actron CP9690 code tester. It checks graphing. It also has emission readiness test. It has other testing abilities. EBAY has it.

[Ryan-J]

Hey amazing video with great detail however im at a loss car was throwing code p0100 maf sensor replaced it engine ran so much smoother no more rough idle but now i get code p0170 fueltrim bank 1 astra h 1.8 2006
LTFT(B1) has a constant 11.7 o2s1 (B1) fluctuate between 0 and negative 25 o2s2 (B1) says negative 100 and doesn’t move any idea where to start i was thinking maybe o2 sensor is bad but im no car expert would be great full for any advice thanks in advance

[strikemi]

I've got a 2004 Volvo v40 2.0t and just got the p0420 code. There isn't any loss of power but I have noticed a smell of unburnt gas or something similar when the car is idling at a stop. I'm pretty sure it's the cat that's gone bad but the fact that there isn't any loss of power concerns me.

[curiouscat]

Great video, full of useful information! If you dont mind me asking some questions I would like to go for it.


1. Is it possible to make sure that the CC is in the place (I mean not stolen or been removed by previous owner) by seeing the voltage on O2 Downstream sensor? I mean would the sensor still give voltage information on OBD2 if the CC is not there?

2. I have Peugeot 407 2.2 petrol. I just bought o2 sensor for downstream because I noticed weird movement (like very slow response) on o2 downstream sensor while checking with OBD2 so Im going to my mechanic to change it this friday but before that could you tell me that P0410 code is related to CC or O2 sensors somehow? My car is keep throwing p0410 code and I already replaced secondary air pump but still get the code. And I have awful gas milage in the city (long ride its normal), is this somehow also related to o2 sensor?


Thanks a lot in advance!

[NobleLock]

Have an 03 Honda Element. Got a P0420, so I set up the O2 sensor readouts on Torque Pro. O2S1 reads fine, but O2S2 is flatlined at 0v no matter what the throttle position. Put in a new O2 sensor and the same. Never any change in the plot, just stays at zero. Would a bad cat cause flatlining, or is this more likely an electrical problem?

[pablovargas]

Hi, my Genesis coupe has a Cat delete and an 02 housing installed. Ofc, because it doesn’t detect different readings with the sensors, throws P0420 code.

I tried doing the spark plug defouler method to avoid the rear 02 being directly in the flow of the exhaust, but it just swaps that code for others.

I want to see if I should clog or open up more the fouler to get the right voltage reading, if it’s reading on idle like 0.3, should I open more or clog the defouler to get it closer to 0.5-7 range?

[jaalvarez]

Hi great video I have same problem you this is the downstream o2 sensor the is bad when I let the pedal off the show rich I press the pedal and the drop o to 10% the problem is when I go like 50 miles and let the pedal is when I have the problem thanks for your time

[RT-idtg]

I got error code P0420 and when analyzing the o2 sensors during a drive, the O2B1S1 was fluctuating as it should but the O2B1S2 would hold at around .75-.8v and then randomly drop to below .2v for a second or two at a time. Compared to the good side, B2S2 which always held pretty constant at around .75-.8v the whole time. Do you think this has to do with the sensor or the catalytic converter?

[thogevoll]

Something you said @2:50 seemed to differ from what you graph said. I was confused. When the switching frequency becomes too low the catalyst efficiency is below threshold. But, low switching frequency means the catalyst's ability to store oxygen is normal.
Did you mean to say if the frequency is too high the efficiency code gets set?

[jorgelopez-ttng]

Very knowledgeable information. Is normal to jump into open loop when descelarate and recover close loop just after step on gas pedal and recovering acceleration? O2S2 both banks stay on 0.8 mV and Fuel trims from that sensors stay on 99%. Any clue is everything normal?

[HSJ-umvw]

Thank you for the video. It was very informative. Currently, I'm facing a problem with my truck throwing p0420 code which mean catalytic below efficiency. When i reviewed the data from both o2 sensors based on your video, my downstream o2 sensor constantly shows voltage reading of .8v while cruising at same speed. It goes down to .2v when i lift foot off gas. Should not it be close to .5 at cruising speed? Do I really need a new catalytic converter?

[georgemalamis]

Hi, and congrats for your video. One question. I have a Mercedes clc 200, i have the primary cat removed which is in the exhaust manifold, and changed the second cat with a high flow 200 cel cat. He installed also an o2 sensor fooler with a minicat in the downstream o2 sensor in order not to have codes. Will i burn more fuel due to downstream o2 sensor fooler or have any other problem? Thanks